The variable “geoplugin_currencyConverter” is the conversion rate for the currency converter base currency.
Like all calls to any of geoPlugin's web services, the default base_currency is USD ($US).
Thus, if your base currency is NOT $US, then you must add the variable base_currency=XXX to the call to geoplugin.net eg

http://www.geoplugin.net/json.gp?base_currency=EUR

Now geoplugin_currencyConverter will output the exchange rate of one Euro for your visitor.

If you are using jQuery for example to do AJAX calls to the JSON webservice, you will probably be seeing the Javascript error Error: Invalid label

This is due to data requests from a server (geoPlugin) in a different domain (your web server). To eliminate this problem, JSONP or “JSON with padding” is required.
To return the JSON results as JSONP, tag jsoncallback=? onto the url when making the jQuery Ajax call to any JSON webservice
eg